It didn't take long for Jethro Tull's Ian Anderson to abandon the jazz and blues elements that characterized his band's 1968 debut album This Was. By the release of 1971's Aqualung, the group had shifted to the European-folk-tinged progressive rock that carried them through more than three dozen albums to follow.

As a band, Jethro Tull ceased operations in 2011; these days, Anderson — who wrote nearly all the group's material, and had always been the face and voice of the band — bills his performances as "Jethro Tull by Ian Anderson." And while the progressive-flavored arrangements are still a key component of the music, for select shows including a May 26 date at Red Rocks, Anderson and band will be joined by an orchestra.

"I've been doing orchestral concerts for about 15 years now," Anderson says. "And it's a fairly well-worn procedure. We have an ever slightly expanding repertoire of music where we have arrangements for the main instruments of the orchestra." But he hesitates to use the term "symphony orchestra," because, as he explains, "there are certain instruments that you would find usually incorporated into a full symphony orchestra — perhaps 60 players — which we don't use."

Anderson runs down a list of instrumentalists who play alongside his five-piece rock band: first and second violins, violas, cellos and contra-bass; a small brass section of two trumpets, two French horns and a trombone; and a similarly scaled-down woodwind section (oboe, bassoon, and flute doubling piccolo). "It's a compact orchestra of a manageable size," Anderson says.

The orchestral musicians allow the arrangements — under direction of Anderson's longtime musical associate John O'Hara — to expand upon the classical motifs that were often hinted at in the original studio recordings. But there remain technical challenges in attempting to combine rock and classical musicians, especially onstage.

"It's not a question of us doing a concert as we would normally do it, and then bolting an orchestra on the side," Anderson emphasizes. "That's usually what rock bands do, which is why you could never hear the orchestra!" For Anderson's orchestral concerts, each instrument is mic'd. "Using open microphones over a group of musicians in a live context is disastrous," he says, "picking up all the sound from everywhere — including the audience — which just then becomes a cacophony."

The situation also requires the rock musicians to alter their playing style, and, even more importantly, their volume. Anderson has a handy reference for his rock musicians when playing with an orchestra: "If you're playing any louder than the first violinist, then you're too loud."

Anderson's attention to detail results in a live mix that balances the orchestral musicians' sound with that of the rock players. He believes it's that delicate balance that many other rock-band-with-orchestra ensembles lack. "With all respect to those who try to do it, they start off thinking somehow they can just do it. For the rock band, it can be another day at the office," he says, "and somehow somebody is going to wave a magic wand and make the symphony orchestra intelligible in the context of a full-blown rock concert. That just doesn't work."